Discharge lamps, specifically HID (high-intensity discharge) lamps are used for a large area of applications where high luminous flux is required. Especially in the automotive field, HID lamps are used as vehicle headlamps.
A discharge lamp comprises a sealed discharge vessel, which may be made e.g. from quartz glass, with an inner discharge space. Two electrodes project into the discharge space, arranged at a distance from each other, to ignite an arc therebetween. The discharge space has a filling comprising a rare gas and further ingredients such as metal halides.
An important aspect today is energy efficiency. The efficiency of a discharge lamp may be measured as lumen output in relation to the electrical power used. In discharge lamps used today for automotive front lighting an efficiency of about 90 lumen per Watt (lm/W) is achieved at a steady state operating power of 35 Watt.
Discharge lamps with lower nominal power, e.g. in the range of 20-30 W, in particular 25 W have already been proposed. However, it is not sufficient to use prior 35 W designs for operation at 25 W, because these show a drastically reduced efficiency if operated at lower power. In order to still deliver sufficient luminous flux for automotive front lighting, HID lamps need to have a special design to yield at the reduced operating power high efficiency.
WO 2009/127993 A1 describes a high pressure gas discharge lamp with a discharge vessel, in which electrodes project into a discharge space of a volume of 12-20 mm3. The discharge space has a filling of a rare gas and a metal halide composition free of mercury. The lamp is intended to operate in steady state operation at an electrical power of 25 W with a luminous flux corresponding to an efficiency of greater than 90 lm/W. In preferred examples, a discharge space is of cylindrical shape and has an inner diameter of 2.2 mm. The discharge vessel is of externally ellipsoid shape with an outer diameter of 5.5 mm. An outer bulb is provided around the discharge vessel filled with a gas filling of reduced pressure to obtain a defined heat transition coefficient. A discharge vessel is filled with Xenon at 15-18 bar cold pressure. A metal halide composition is contained in the discharge space, comprising in a first example only NaI and ScI3 and in further examples additionally ThI4. The metal halides are provided in a quantity of 15.8 μg/μl and 10.52 μg/μl.
In discharge lamps today, there is an increasing demand for lamps delivering light at high color temperature, such as up to 5000 K. Thus, lamp designs intended for high efficiency at reduced power would have to be redesigned to deliver the required color temperature light also.